Shellfish feel impact of more acid seas

January 22nd, 2014, by Tim Radford

Low oxygen and rising acidity in the oceans spell trouble for some species of scallopImage: By Jeremy Keith from Brighton & Hove, UK, via Wikimedia Commons

FOR IMMEDIATE RELEASEResearchers in the US have discovered that several more marine species are being damaged by the effects of the increasing acidity of the oceans, a direct consequence of greenhouse gas emissions.LONDON, 22 January – Ocean acidification brings fresh problems for Californian native oysters. Like some creature from a horror movie, a driller killer threatens Ostrea lurida, the Olympia oyster that dwells in the estuaries of western North America. Many species are likely to face problems as pH levels (which measure how acid a liquid is) change and ocean chemistry begins to alter as the world warms and ever more dissolved carbon dioxide flows into the sea and adds to its acidity. Researchers have observed that coral skeletons are affected and larval oysters find it more difficult to build their first shell structures. The change towards greater acidity seems to trigger learning difficulties in juvenile rockfish and make it harder for the conch snail to leap out of the way of a predator’s poisoned dart. And three separate research papers bring yet more bad news for yet more sea creatures. Eric Sanford and colleagues at the University of California, Davis, report in the Proceedings of the Royal Society B that under more acid water conditions, the Olympia oyster experiences a 20% increase in drilling predation. Researchers conducted a direct experiment involving oysters reared in normal conditions, oysters reared in water high in dissolved carbon dioxide, and an invasive predator from a distant ocean called Urosalpinx cinerea, the Atlantic oyster drill. Their assumption was that bivalves (creatures with a hinged shell) in more acid water would grow thinner shells, and that drilling predators would selectively choose the victims that would be easiest to drill into.

Problems multiply

It didn’t work quite like that – the experimental oysters did not have thinner shells. But these oysters were victims all the same. They were 30% to 40% smaller than the control group of oysters in the other tank “and these smaller individuals were consumed at disproportionately greater rates”, the authors say. The invasive snails, on the other hand, were not bothered by the change in water pH. The results indicate, say the researchers, that ocean acidification “can negatively affect the early life stages of Olympia oysters.” They have been subjected already to overfishing, disease, habitat loss, pollution and hypoxia, when water is so rich in nutrients that it becomes starved of oxygen and turns into a dead zone where nothing much can survive for long. Extra vulnerability to an invasive driller killer is, the scientists carefully say in non-emotive language, “a relatively novel stressor for this species.” Hypoxia, too, turns out to be a problem made worse by carbon dioxide. Low oxygen waters are already acidified waters, say Christopher Gobler of Stony Brook University in the US and colleagues in the Public Library of Science journal PLOS One. They report that a combination of low oxygen and low pH led to higher rates of death and slower growth for young bay scallops and hard clams than expected from either individual factor. “Low oxygen zones in coastal and open ocean ecosystems have expanded in recent decades, a trend that will accelerate with climatic warming”, says Gobler.

Threat to algae

“There is a growing recognition that low oxygen regions of the ocean are also acidified, a condition that will intensify with rising levels of atmospheric CO2 due to the burning of fossil fuels causing ocean acidification. Hence the low oxygen, low pH conditions used in this study will be increasingly common in the world’s oceans in the future.” And ocean acidification is not just making life good for predators and bad for the prey, it could be threatening to alter the basic biodiversity of the sea. Sophie McCoy of the University of Chicago reports in Ecology Letters that she and Cathy Pfister looked at the dynamics of coralline algae that live around Tatoosh Island, Washington, on the Pacific coast of the US. These little creatures, like oysters, grow calcium carbonate skeletons. In previous observations in which four species were transplanted to these waters, one species called Pseudolithophyllum muricatum emerged as the undisputed winner. In the 1980s, its skeleton grew twice as thick as its competitors’. In the latest round of tests, there was no clear winner: no species was dominant, and P. muricatum won less than 25% of the time – a response, the authors think, to changes in the pH of the sea water just in the last 12 years. The total energy available to the organisms was the same, but their responses were different: those that needed to make more calcium carbonate tissue were under more stress than those that did not. This experiment was a “real world” test rather than a laboratory experiment. “Field sites like Tatoosh are unique because we have a lot of historical ecological data going back decades,” said McCoy. “I think it is really important to use that in nature to understand what is going on.” – Climate News Network

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